1. Field of the Invention
The present invention relates to a power recovery apparatus for use in a seawater desalination plant or a seawater desalination system for desalinating seawater by removing salinity from the seawater. Particularly, the present invention relates to a power recovery chamber which is preferably used for a positive-displacement power recovery apparatus in the seawater desalination plant or system, can improve reliability of the seawater desalination plant or system, can operate the seawater desalination plant or system at high efficiency (energy-saving), and can recover fresh water by the seawater desalination plant or system at high efficiency.
2. Description of the Related Art
A seawater desalination plant which employs a reverse osmosis membrane method is composed mainly of a pretreatment system, a high-pressure pump, a reverse osmosis membrane cartridge, and a power recovery apparatus. When seawater is introduced into the seawater desalination plant, the seawater is processed to have certain water qualities by the pretreatment system, and then delivered into the reverse osmosis membrane cartridge under pressure by the high-pressure pump. Part of the high-pressure seawater in the reverse osmosis membrane cartridge passes through the reverse osmosis membrane against the reverse osmosis pressure and is desalinated, and fresh water is taken out from the reverse osmosis membrane cartridge. The remaining concentrated seawater with a high salt content is discharged as a reject from the reverse osmosis membrane cartridge. The largest operational cost (electric expenses) in the seawater desalination plant is energy cost for pressurizing the pretreated seawater up to such a pressure as to overcome the osmotic pressure, i.e. up to the reverse osmosis pressure. That is, the operational cost of the seawater desalination plant depends heavily on pressuring energy of the seawater by the high-pressure pump.
Specifically, more than half of the electric expenses as the highest cost in the seawater desalination plant are consumed to operate the high-pressure pump for pressuring the seawater. Therefore, the power recovery apparatus for effectively recovering the pressure energy from the high-pressure reject with the high salt content which has been discharged from the reverse osmosis membrane cartridge has a significant role.
FIG. 22 is a schematic view showing an example of a seawater desalination plant which employs a reverse osmosis membrane method. As shown in FIG. 22, when seawater 1 is pumped into the seawater desalination plant by an intake pump 2, the seawater 1 is processed to have certain water qualities by a pretreatment system 3, and then pressurized and delivered via a high-pressure line 7 into a reverse osmosis membrane cartridge 8 by a high-pressure pump 5 that is driven by an electric motor 6. Part of the seawater in a high-pressure chamber 9 of the reverse osmosis membrane cartridge 8 passes through a reverse osmosis membrane 10 against the reverse osmosis pressure and is desalinated, and then desalinated water 12 is taken out from the reverse osmosis membrane cartridge 8. The remaining concentrated seawater with a high salt content is discharged under pressure as high-pressure reject 13 from the reverse osmosis membrane cartridge 8 into a concentrated seawater line. The high-pressure reject (high-pressure concentrated seawater) 13 discharged from the reverse osmosis membrane cartridge 8 is introduced into a power recovery apparatus 23.
The power recovery apparatus 23 utilizes a positive-displacement power recovery apparatus as a measure (system) for operating the seawater desalination plant at high efficiency by effectively recovering and utilizing pressure energy of the high-pressure reject 13 with the high salt content.
Examples of conventional positive-displacement power recovery apparatuses are disclosed in U.S. Pat. Nos. 5,306,428 and 5,797,429.
FIG. 23 is a schematic view showing an example of a conventional positive-displacement power recovery apparatus. The positive-displacement power recovery apparatus is mainly composed of a directional control valve 20, two power recovery chambers 21, and a check valve module 22.
The function of the positive-displacement power recovery apparatus is as follows:
(1) The high-pressure reject 13 from the reverse osmosis membrane cartridge 8 is introduced into the directional control valve 20.
(2) The high-pressure reject 13 is introduced alternately into the two power recovery chambers 21 by actuation of the directional control valve 20.
(3) The piston in the power recovery chamber 21 is driven.
(4) Seawater which has been introduced from the supply line 4 through the check valve module 22 into the power recovery chambers 21 is pressurized by driving of the piston.
(5) The seawater which has been pressurized in the power recovery chambers 21 is discharged through the check valve module 22 to the supply seawater bypass boost line 24, and is then introduced into the booster pump 27 which is driven by the electric motor 26. The reference numeral 25 represents a discharge line.
Because the positive-displacement power recovery apparatus is used in the seawater desalination plant, the flow rate of the pretreated seawater which is pressurized by the high-pressure pump can be reduced, and energy (flow rate, pressure) required for operating the seawater desalination plant can be reduced, resulting in high operational efficiency of the system.
FIG. 24 is a schematic view showing an example of the conventional power recovery chamber. As shown in FIG. 24, the power recovery chamber 21 comprises a cylinder 31 having a cylindrical shape, and a piston 33 which is reciprocated in the cylinder 31. The cylinder 31 has two intake and discharge ports 31a, 31b. The piston 33 is arranged to be movable in an axial direction in the cylinder 31.
The function of the power recovery chamber 21 is as follows:
(1) The piston 33 is driven by pressure of the high-pressure reject 13 introduced through the directional control valve 20 into one side of the power recovery chamber 21 to boost the seawater introduced by the intake pump 2 into the other side of the power recovery chamber 21.
(2) The piston 33 is driven by discharge pressure of the intake pump 2 to discharge the reject which has been introduced into one side of the power recovery chambers 21 through the directional control valve 20 to the discharge line 25.
Specifically, in the power recovery chamber 21, the following cycle is performed:
(1) Introduction of the seawater →(2) Driving of the piston by introduction of the high-pressure reject→(3) Boosting of the seawater→(1) Introduction of the seawater
That is, the cycle of (1)→(2)→(3) is repeated to perform introduction of the fluid and discharge of the fluid.
The cycle of (1)→(2)→(3) may be expressed in different words as follows:
(A) In FIG. 24, when the piston 33 moves from the left end to the right end of the cylinder 31, introduction of the seawater and discharge of the concentrated seawater (reject) are performed.
(B) When the piston 33 moves from the right end to the left end of the cylinder 31, the seawater is boosted by introduction of the high-pressure concentrated seawater (high-pressure reject).
(C) The two cylinders 31 alternate between (A) and (B) to recover the power of the high-pressure concentrated seawater having a certain pressure and a certain flow rate in the manner in which the seawater having a constant flow rate is boosted.
The conventional power recovery chamber of the positive-displacement power recovery apparatus as typified by U.S. Pat. Nos. 5,306,428 and 5,797,429 has the following disadvantages:
(1) In the conventional power recovery chamber, the outer circumferential surface of the piston is brought into sliding contact with the inner circumferential surface of the cylinder. In particular, in the power recovery chamber formed for the purpose of processing the large flow rate, the area of the sliding surface of the piston (in proportion to the diameter of the piston) and the range of reciprocating motion of the piston (stroke) become large. As an example of the dimension of the power recovery chamber, the inner diameter of the cylinder (≈ outer diameter of the piston) is about 0.4 m, and the length of the chamber is about 7 m. As is apparent from this example, the power recovery chamber is large in size, and it is highly likely that frictional wear (wear caused by friction) is generated in one of the sliding surfaces of the cylinder and the piston or both of the sliding surfaces of the cylinder and the piston.
Further, the incidence rate of the frictional wear is greatly related to sliding area A, sliding velocity V, and contact pressure P. Specifically, as sliding area A or contact pressure P in the sliding portion is larger and sliding velocity V is higher, the progression rate and the incidence rate of the wear increases. Abrasion powder is produced by the frictional wear. Specifically, as each of sliding area A, sliding velocity V, and contact pressure P is larger, the abrasion powder increases in quantity (hereinafter, this relation is referred to as “relation 1”).
(2) The friction loss generated at the sliding surfaces when the piston is moved in the power recovery chamber is related to the sliding area A of the piston which is brought in sliding contact with the inner surface of the cylinder. As the sliding area A is larger, the friction loss increases. The increase of the friction loss causes a decrease of pressure rising of the seawater in the power recovery chamber, and thus the required energy recovery efficiency cannot be obtained. That is, the larger the sliding area A is, the larger the friction loss is (hereinafter, this relation is referred to as “relation 2”).
PV value is used as general parameter of frictional wear condition. The PV value is expressed by the product of contact pressure P and sliding velocity V. The larger the PV value is, the larger the friction loss of the sliding part is, and the larger the generation of abrasion powder is. Specifically, the larger the PV value as the general parameter is, the larger the friction loss of the sliding part is, and the larger the generation of abrasion powder is (hereinafter, this relation is referred to as “relation 3”).
As presented above, in the power recovery chamber having a predetermined length in a thrust direction (axial direction of cylinder) and having a piston, if the flow rate of fluid to be handled is the same, the following relationship is established.
(1) If the inner diameter of the cylinder (≈outer diameter of the piston) is large, the sliding velocity V becomes low, and the sliding area A becomes large.
(2) If the inner diameter of the cylinder (≈outer diameter of the piston) is small, the sliding velocity V becomes high, and the sliding area A becomes small.
Further, the relationship between the sliding area A and the friction loss or the generation of abrasion powder is summarized as follows:
Sliding area A; large→i) friction loss; large, ii) generation of abrasion powder; large
Sliding area A; small→i) friction loss; small, ii) generation of abrasion powder; small
Further, if the sliding velocity V is high (or low) as indicated in the relation 1, the generation of abrasion powder becomes large (or small).
Specifically, in the conventional power recovery chamber, trade-off (antinomic phenomenon) between the sliding velocity V and the sliding area A cannot be solved, and thus the problems described in the above (1) and (2) are presented under existing circumstances.
Specifically, the problems are summarized as follows:
(A) Because either the sliding velocity V or the sliding area A becomes large, the problem of “wear” on the basis of “relation 1” cannot be solved. (B) With respect to the problem of “friction”, if the sliding area A is small, even if the sliding velocity V becomes large (high), the problem of “friction” can be improved if only the above “relation 2” is established. However, because there is “relation 3”, even if the sliding area A becomes small, the problem of “friction” cannot be improved.
Accordingly, abrasion powder of one of the cylinder and the piston or both of the cylinder and the piston produced by the above frictional wear enters constituent equipment of the system, and the performance of the constituent equipment deteriorates rapidly to shorten the service life of the system or to cause impairment of the constituent equipment. Consequently, it is highly likely that the operation of the system is shut down. In particular, the seawater desalination system or plant is closely tied with people's daily lives, and thus a serious problem arises when the seawater desalination system has some trouble.
(3) In order to avoid the above problems (1) and (2), in some cases, the combination of materials having low frictional wear characteristic under lubrication of seawater is applied to the cylinder and the piston. As an example of such materials, there are ceramics, resin, engineering plastics, and the like. However, because the chamber is very large in size as in the above example, it is difficult to obtain machined (or worked) components having suitable size. In particular, it is likely to exceed the limits of injection molding or coating treatment applied to different materials. Some materials are difficult to obtain because of size limitations.
If ceramics are used and processed, the cost of such processing is very expensive.
(4) As a measure for avoiding or suppressing the above problems (1) and (2), it is possible to make surface roughness of the sliding surfaces of the cylinder and the piston smooth (to make surface roughness small).
However, because the power recovery chamber is very large in size as in the above example, it is difficult to machine or process such chamber, and the cost of such processing Is very expensive.
(5) In order to avoid the above problem (2), i.e. the problem of friction loss, it is conceivable that the power recovery chamber having the form of no piston is applied to the seawater desalination system.
FIG. 25 is a schematic view showing an example of a power recovery chamber having the form of no piston. As shown in FIG. 25, a power recovery chamber 41 is in the form of closed container. The power recovery chamber 41 has two inlet and outlet ports 41a, 41b, and introduction or discharge of the concentrated seawater and the seawater to or from the power recovery chamber 41 is performed through the inlet and outlet ports 41a, 41b. In the power recovery chamber 41, a contact interface CI between the concentrated seawater and the seawater moves in the power recovery chamber 41 by pressure balance between the concentrated seawater and the seawater. Specifically, in the power recovery chamber 41, the contact interface CI serves as the piston 33 of the power recovery chamber 21 shown in FIG. 24.
The power recovery chamber 41 shown in FIG. 25 has a problem that salt content of the intake seawater becomes high in the power recovery chamber 41 by mixing of the concentrated seawater and the seawater at the contact interface CI. Thus, when the seawater pressurized in the chamber 41, i.e. the pressurized seawater and the intake seawater from the high pressure line are merged and introduced into the reverse osmosis membrane cartridge, salt content of the merged seawater becomes high. Accordingly, the freshwater recovery rate of the reverse osmosis membrane cartridge is lowered, and the service life of the reverse osmosis membrane cartridge is shortened.
Further, in order to keep the freshwater recovery rate of the reverse osmosis membrane, the introduction pressure of the seawater introduced into the reverse osmosis membrane cartridge (pressurizing rate by the high-pressure pump) is required to be higher than normal, resulting in lowering the operational efficiency of the system.
In addition, it is conceivable to make the diameter of the power recovery chamber smaller, thereby reducing contact area of the contact interface. However, in order to ensure the required flow rate of the intake seawater to be pressurized in the chamber, it is necessary to increase operation speed of the contact interface by a magnitude corresponding to the reduced diameter of the chamber. Therefore, it is necessary to increase switching operation speed of the directional control valve (see the directional control valve 20 shown in FIG. 23), and thus energy consumed by actuation of the directional control valve increases, resulting in increasing electric energy required for operation of the system.
Further, in addition to the above problems, when the directional control valve is stopped due to some trouble including malfunction, at least one contact interface of the two chambers disappears after such contact interface moves to the check valve module side. Thus, the chamber having no contact interface becomes a passage of the high-pressure concentrated seawater, and the concentrated seawater having high salt content merges into the intake seawater from the high-pressure pump, and is then introduced into the reverse osmosis membrane cartridge. Consequently, the freshwater recovery rate of the reverse osmosis membrane is lowered, and the service life of the reverse osmosis membrane cartridge is shortened.
(6) As equipment having the same function as the power recovery chamber, there is a double-acting double-rod cylinder. Therefore, it is conceivable to apply this double-acting double-rod cylinder to the seawater desalination system.
FIG. 26 is a schematic view showing an example of a power recovery chamber which employs a double-acting double-rod cylinder. As shown in FIG. 26, a power recovery chamber 51 comprises a cylinder 61 having a cylindrical shape, a piston 62 which is reciprocated in the cylinder 61, and piston rods 63 extending from both sides of the piston 62 in an axial direction. The chamber 51 has two intake and discharge ports 51a, 51b. The piston 62 is arranged so as to move in an axial direction in the cylinder 61 together with the piston rods 63.
As shown in FIG. 26, at least two packings 64 are mounted on the piston 62. In the case where the double-acting double-rod cylinder is used as the power recovery chamber, the packings 64 are worn or deteriorated, and therefore there is a problem that the power recovery efficiency is lowered because of leakage of working fluid, and the service life of the equipment is shortened.